# A study of gamma-ray burst afterglows as they first come into view of the Swift Ultraviolet and Optical Telescope

## TL;DR

This study analyzes the initial UVOT observations of gamma-ray burst afterglows, confirming data quality and exploring early optical behavior, revealing that some afterglows peak after the initial settling period and challenging thick-shell models.

## Contribution

It provides the first detailed analysis of UVOT settling data for GRB afterglows, demonstrating detection capabilities and early optical peak characteristics.

## Key findings

- All 23 bright afterglows were detected during settling exposures.
- Some afterglows peaked after the settling period, with significant optical rises.
- The optical rise behavior challenges thick-shell afterglow models.

## Abstract

We examine the the emission from optically bright gamma-ray burst (GRB) afterglows as the Ultraviolet and Optical Telescope (UVOT) on the Neil Gehrels Swift Observatory first begins observing, following the slew to target the GRB, while the pointing of the Swift satellite is still settling. We verify the photometric quality of the UVOT settling data using bright stars in the field of view. In the majority of cases we find no problems with the settling exposure photometry, but in one case we excise the first second of the exposure to mitigate a spacecraft attitude reconstruction issue, and in a second case we exclude the first second of the exposure in which the UVOT photocathode voltage appears to be ramping up. Of a sample of 23 afterglows which have peak V magnitudes <16, we find that all are detected in the settling exposures, when Swift arrives on target. For 9 of the GRBs the UVOT settling exposure took place before the conclusion of the prompt gamma-ray emission. Five of these GRBs have well defined optical peaks after the settling exposures, with rises of >0.5 mag in their optical lightcurves, and there is a marginal trend for these GRBs to have long T90. Such a trend is expected for thick-shell afterglows, but the temporal indices of the optical rises and the timing of the optical peaks appear to rule out thick shells.